Sverre Planke2 and Herve
Cambray3

ABSTRACT

Ocean Drilling Program Hole 917A penetrated 779 m of
subaerially emplaced basalts and dacites near the
landward edge of the seaward-dipping reflector sequences
on the southeast Greenland volcanic margin. Wireline logs
were recorded in a 430-m interval covering four very thin
sediment units and 48 lava units with mean and maximum
thickness of 8.9 m and 53 m, respectively. Seismic
compressional-, shear-, and tube-wave velocities were
obtained by slowness-time coherency inversion of sonic
waveform data. The compressional-wave velocity log shows
asymmetric cyclic variations, with velocities of 2.5 to
5.5 km/s in the brecciated and vesicular flow tops, and
high velocities, 5 to 6 km/s, in the massive and
fractured central and lower part of the lavas. The
variations in velocity are attributed to systematic
changes in total porosity, pore geometry, and alteration.
The shear-wave velocity log recorded in high-velocity (Vs
> 1.5 km/s) intervals correlate well with the
compressional-wave velocity log. High shear-wave
amplitudes identified near numerous high-impedance
boundaries are related to mode-conversion within the lava
pile. Vp/Vs
ratios of 1.8 to 2.0 are recorded throughout the
sequence, with no systematic variations. The normalized
tube-wave energy log is broadly inversely proportional to
the velocity logs, with low energy values in fractured
massive intervals and high energy values near unit
boundaries and within the top part of the lavas.
Comparison of conventional logs and the Formation
MicroScanner image gives high confidence to the
usefulness of conventional logs in terms of recording
downhole textural and lithologic variations in flood
basalt terrains. The average velocity in the lava pile is
4.17 km/s, corresponding to 4.05 km/s obtained from the
interpretation of seismic reflection data, and is
primarily a function of the average lava unit thickness.